An introduction to Capacitive Sensing—Part II

Editor’s Note:  Find Part I here

When it comes to capacitive sensing design, layout plays a crucial role. Giving importance to layout not only aids in superior performance (lower noise and higher signal) but also helps in achieving EMI/EMC compliance. It should be kept in mind that a good layout helps in realizing the following two objectives:

• Higher finger capacitance and lower parasitic capacitance: The signal in a given system is the sum of signals due to parasitic capacitance and those due to finger capacitance. It is important to reduce parasitic capacitance because, in order to increase a particular signal within a fixed range, the other signal must always be reduced to avoid saturating the signal.
• Lower noise: This helps in making the system more reliable and avoid false detects.

Some of the key guidelines for schematic and layout include:

Sensor shape and size

Capacitive sensing sensors are in demand because of their innovative shape, which gives the design a unique and slender look. Understanding what imposes the limitations on selecting a particular shape is important because it affects the performance parameters of the design. Sensors laid on a PCB can be divided into four different categories:

Independent sensors or buttons

The most widely used shape is the button, which is either circular or square.  However, designers can chose their own shapes, depending upon the board area and other design constraints. This shape is recommended from silicon manufacturers because of the geometry of a finger is also somewhat circular in nature.

An important consideration when choosing a sensor shape is to avoid sharp corners:

• Sharp corners are more sensitive and hence the sensor response w.r.t touch position/direction would be non-linear. This is an unintended behavior
• Corners radiate more EMI, which may cause compliance issues.  Corners should be rounded wherever required.

Figure 1 shows some of the recommended patterns.

The size of the sensor determines the amount of finger capacitance. For a better SNR, finger capacitance should be as high as possible but because of design constraints like overlay thickness, the type of materials used, etc., finger capacitance typically lies in the range of 0.1pF to 1pF.

When a user touches the sensor, the finger acts like the second electrode of a parallel plate capacitor, the sensor pad being the first. Thus, the following formula gives a fair idea of how the sensor size affects the finger capacitance:

Figure 2 shows the finger capacitance with the change in size of the button. One can safely assume that by increasing the sensor area, finger capacitance can be increased but increasing it more than the finger size will not have any effect because the maximum area will always be limited by the area of the second plate; i.e., the finger.

Proximity sensors

Proximity sensors are generally used for lowering the power consumption in any application. The sensing device can be put to sleep and a proximity sensor can be implemented to sense the approach of hand towards the keypad, thus activating the required functions like backlighting and keypad scanning.

Shape and size:

Proximity sensing requires that the electric field be projected to much larger distances than buttons or sliders. This demands the sensor area be large; however, there is still a constraint on sensor size imposed by the parasitic capacitance of the sensor that should be as low as possible. This necessitates the implementation of the sensor in such a way that we get high electric field strength at larger distances while keeping the sensor area as small as possible. Loops with trace thickness of 2-3 mm have proved to be the most successful implementation by far.  The rule of thumb for such loops is to have the loop diagonal/diameter equal to the proximity distance required.